1. Field of the Invention
The present invention relates to the preparation of homogeneous copolyamides having both high melting points and good flexibility, and, more especially, to such homogeneous copolyamides produced from hexamethylenediamine, either adipic acid alone or admixture thereof with at least one other lower or short-chain dicarboxylic acid and a fatty acid dimer, the amount of short-chain diacid(s) relative to the total amount of the acids present [short-chain acid(s)+dimer acid] ranging from 50 to 99 mole %.
2. Description of the Prior Art
Copolyamides of the aforesaid general type, based on hexamethylenediamine, adipic acid and dimer acid, are described in French Pat. No. 2,459,810, assigned to the assignee thereof. As utilized herein, by "homogeneous copolyamide" is intended a copolymer which forms a single phase in the melt state or a single amorphous phase in the solid state, as can be determined, for example, by scanning electron microscopy which reveals any dispersed phase which may be present, by fixing same with phosphotungstic acid.
A much simpler means for determining this homogeneity consists in observing the state of transparency, on the one hand, of a melt having a thickness of approximately 0.2 to 0.5 cm, heated to a temperature of at least 10.degree. C. above its melting point and, on the other hand, of a small diameter rod or film obtained from such melt after rapid cooling, for example, by immersion in water at ambient temperature. An absolutely or completely transparent rod or film indicates perfect homogeneity of the polymer. When the film is greater in thickness or when the cooling has been slower, a translucent zone may indicate a certain degree of crystallinity; a white film incorporating streaks or inclusions (fisheyes, for example) is the sign of a major heterogeneity. This homogeneity signals the fact that the copolymers in question have thermomechanical properties which can be readily reproduced, in addition to good transparency. Also as utilized herein, by a copolyamide having "good flexibility" is intended a copolyamide, the flexural modulus of which is significantly lower than that of the conventional polyamides, such as polyhexamethylene adipate (nylon 66) or polycaprolactam (nylon 6) and is on the order of, or lower than, that of the polyamides considered to be flexible, such as, for example, polyundecanamide (nylon 11).
In the copolyamides of the type of the aforesaid prior art, the crystallinity and, consequently, the ability to obtain high melting points and good thermomechanical properties, are essentially contributed by the group of segments which are derived from the condensation of the short-chain acid molecules (adipic acid) with a portion of the hexamethylenediamine molecules. The ability to have flexibility and good elasticity is substantially contributed by the group of segments which are derived from the condensation of the fatty acid dimer molecules with the other portion of the hexamethylenediamine molecules. It is apparent that, by changing the quantity of short-chain acid relative to the total amount of the acids present in the range from 50 to 99 mole %, copolyamides are obtained, the melting points of which will be more or less high and the flexibility of which will be more or less. The copolyamides in which the amount of short-chain acid is in the range of from 70 to 90 mole % are of particular interest because they have both high melting points and excellent flexibility.
A variety of specific processes enabling the preparation of such homogeneous copolyamides are also described in the aforementioned French patent. According to a first process, the following steps are carried out:
(1) prepolymers are prepared in a heterogeneous medium from a mixture of adipic acid, dimer acid and hexamethylenediamine, or from their corresponding salts, the amounts of the constituents of the beginning mixture being such that the quantity of adipic acid relative to the total acids is in the range of from 50 to 99 mole % and that the contents of amino groups and of carboxylic groups do not differ by more than 5% as an absolute value, the polymerization reaction consisting of adjusting the starting mixture up to a temperature of 270.degree. C. by heating it in a gradual and uniform manner, over a period of time ranging from 30 minutes to several hours, at a water vapor pressure of from 1.3 to 2.5 MPa, which is the chemical equilibrium state corresponding to commencement of the following phase (2);
(2) a homogenization phase is then carried out for a period of from 0.5 to 5 hours, at a temperature of from 270.degree. to 290.degree. C. under a water vapor pressure of from 1.3 to 2.5 MPa, during which the equilibrium state of the amidification reactions remains unchanged; and
(3) lastly, a polycondensation is conducted such as to convert the homogeneous prepolymers thus produced into the desired copolyamides, the polycondensation reaction being carried out in a conventional manner at a temperature of from 260.degree. to 290.degree. C. at atmospheric pressure or at a lower pressure, for a period of time ranging from 30 minutes to several hours.
In a second such process, the following steps are carried out:
(1) prepolymers are prepared in a heterogeneous medium from a mixture of adipic acid, dimer acid and hexamethylenediamine, or of the corresponding salts thereof, the amounts of the constituents of the starting mixture being such that the quantity of adipic acid relative to the total acids is in the range of from 50 to 99 mole % and that the contents of amino groups and of carboxylic groups differ by more than 5% as an absolute value, the prepolymerization reaction consisting of adjusting the temperature of the starting mixture to a temperature corresponding to the beginning of the following phase (2) in a gradual and uniform manner, over a period of time ranging from 30 minutes to several hours;
(2) a homogenization phase is then carried out for a period of time of from 10 minutes to 2 hours at a temperature of from 265.degree. to 290.degree. C.; and
(3) lastly, the gradual addition of the deficient reactant is effected in a conventional manner and the polycondensation is completed at temperatures of from 260.degree. to 290.degree. C. at atmospheric pressure, or at a lower pressure, for a period of time ranging from 30 minutes to several hours until the desired copolyamides are produced.
Lastly, in a third process, copolymers are directly produced at a temperature of from 150.degree. to 300.degree. C., in a homogeneous medium, from a mixture of adipic acid, dimer acid and hexamethylenediamine, the amount of adipic acid relative to the amount of total acids being in the range of from 50 to 99 mole % and the contents of amino groups and of carboxylic groups not differing by more than 5% as an absolute value, the medium being rendered homogeneous by the use of a third solvent for the mixture of the three starting materials or of the corresponding salts and/or the oligomers of adipic acid and hexamethylenediamine and dimer acid and hexamethylenediamine having a molecular weight of below 5,000, the said solvent being inert vis-a-vis the amidification reactants and reactions.